For such a seat slide devices for a vehicle, there exist a manual seat slide device which can be locked or unlocked by a seating passenger manually to move an unlocked seat to a desired position in a front-rear direction of the vehicle and an electrical seat slide device which moves a seat to a desired position in the front-rear direction of the vehicle by forward and reverse rotations of a motor. Again, each of the manual seat slide device and the electrical seat slide device includes a lower rail fixed to a vehicle body, an upper rail fixed on an under face of a seat and adapted so as to be movable with respect to the lower rail in the longitudinal direction and, a friction reducing member for reducing friction between the lower rail and the upper rail.
In a seat slide device of the conventional example, either a resinous shown in JP 2006-168589 A or guide balls rotatably supported by a retainer shown in JP 2010-105618 A is mainly employed as the friction reducing member.